1. Field of the Invention
The present invention relates to a quartz crystal device, such as a crystal unit and a crystal oscillator, having the configuration in which at least a vibration region of a quartz crystal blank is hermetically encapsulated in a container.
2. Description of the Related Art
A quartz crystal unit in which a quartz crystal blank is hermetically encapsulated in a container, a crystal oscillator in which such a crystal unit and an IC (integrated circuit) chip with a circuit using the crystal unit are integrated, and the like are generically called quartz crystal devices, and are used in various kinds of electronic equipment. Above all, surface-mount quartz crystal devices each having the configuration in which a crystal blank is hermetically encapsulated in a surface-mount container are widely incorporated especially in portable electronic equipment, for example, portable telephones as reference sources for frequency and time because of their compactness and light weight.
FIGS. 1A and 1B are respectively a sectional view and a bottom view showing a configuration of a conventional surface-mount crystal oscillator as one example of a quartz crystal device.
The illustrated crystal unit is such that quartz crystal blank 2 is housed in surface-mount container body 1, and crystal blank 2 is hermetically encapsulated in the container by being covered with metal cover 3. Container body 1 is constituted of, for example, laminated ceramics, and has a substantially rectangular planar outer shape, that is, a flat outer shape of a substantially rectangular parallelepiped which looks like a rectangle seen from above when the crystal unit is mounted on a wiring board. A recess for housing crystal blank 2 is formed on a top surface of container body 1. On an inner bottom surface of the recess, a pair of crystal holding terminals 4 are provided to be close to the positions of both ends of one side of the inner bottom surface. Crystal holding terminals 4 are used for electrically and mechanically holding crystal blank 2 in the recess as will be described later.
Mounting electrodes which are used when container body 1 is surface-mounted on a wiring substrate are provided at four corner portions on an outer bottom surface of container body 1, that is, a surface which faces the wiring board when being mounted on the wiring board. Each of the mounting electrodes is formed as a substantially rectangular conductive layer. Out of these four mounting electrodes, a pair of mounting electrodes 5a which are located at both ends of one diagonal line in the outer bottom surface of container body 1 are electrically connected to a pair of crystal holding terminals 4 through conductive paths formed in container body 1. Further, remaining two mounting electrodes 5b are used as ground terminals.
Crystal blank 2 which are used in such a crystal unit is constituted of, for example, a substantially rectangular AT-cut quartz crystal blank, and excitation electrodes 6a are formed respectively on both principal surfaces. Lead electrodes 6b are extended from a pair of excitation electrodes 6a to both sides of one end portion of crystal blank 2. Lead electrode 6b is formed to be folded back between both the principal surfaces of crystal blank 2 at the position of the end portion of crystal blank 2. Crystal blank 2 is fixed to and held in the recess of container body 1 and electrically and mechanically connected to container body 1 by fixing these lead electrodes 6b to crystal holding terminals 4 with, for example, conductive adhesive 7 or the like at the positions where a pair of lead electrodes 6b are led. Further, the mass of crystal blank 2 is supported by conductive adhesive 7. Here, as conductive adhesive 7, the conductive adhesive which is relatively rich in flexibility even after cured is used, in order to prevent a frequency variation from being caused as a result of stress being exerted on crystal blank 2 by residual stress at the time of curing of the adhesive. As a conductive adhesive having such a characteristic, for example, a silicone-based conductive adhesive is cited.
A metal thick film or metal ring 8 is provided on the top surface of container body 1 to surround an opening by the recess, and metal cover 3 is joined to metal ring 8 by seam welding or beam welding. Metal ring 8 is electrically connected to mounting electrodes 5b used as the ground terminals via through-holes formed in container body 1.
Such a surface-mount crystal unit is generally mounted on a wiring board by reflow soldering. In the crystal unit, laminated ceramics for which through-hole machining, formation of an interlayer conductive layer and the like are easily performed are used for the container body, and therefore, leading of the electrodes from the crystal blank can be easily performed.
Japanese Patent Laid-Open No. 9-326663 (JP-A-9-326663) discloses the crystal unit with the configuration in which covers are joined to a quartz crystal plate used as a vibration plate from above and below, and as a result, the vibration portion of the quartz crystal plate is hermetically encapsulated in a space formed by the upper and lower covers. In the crystal unit of JP-A-9-326663, as the upper and lower covers, the ones each constituted of quartz crystal are used, and these covers are joined to the quartz crystal plate used as the vibration plate by direct bonding. The direct bonding means that both members are joined in the form in which the atoms on the surface of one member and the atoms on the surface of the other member are chemically coupled. In the direct bonding of two members each constituted of quartz crystal, siloxane bond (Si—O—Si) is generally formed between both the members.
In the conventional crystal unit shown in FIGS. 1A and 1B, the crystal blank is fixed to the crystal holding terminals with, for example, the conductive adhesive or the like, but in such a configuration, when a mechanical impact is applied to the crystal unit, the impact which is applied to the crystal blank is directly transmitted to the conductive adhesive, and therefore, there is the fear that the conductive adhesive is torn off and peeled from the crystal holding terminals, or stress occurs to the conductive adhesive. When the conductive adhesive peels off, the crystal unit does not function as a crystal unit, and when stress occurs to the conductive adhesive, the characteristic of the system which mechanically holds the crystal blank varies, and therefore, the influence is exerted on the crystal blank and is likely to change the vibration characteristic and the resonance frequency of the crystal blank. Further, due to a variation in the inclination of the crystal blank in the container body, a free end of the crystal blank is likely to contact the container body or the metal cover, and in order to prevent such contact, the recess of the container body needs to be formed in a certain size or more.
On the other hand, in the configuration disclosed in JP-A-9-326663, the covers are joined to the quartz crystal plate which functions as the vibration plate by direct bonding, and therefore, occurrence of a trouble due to the conductive adhesive as described above when a mechanical impact is applied to the crystal unit is prevented, but since a special joining method called direct bonding is used, there are the problems that manufacture of the crystal unit is difficult and manufacture cost increases. Further, as the covers, only the covers made of quartz crystal or glass can be used, and therefore, there is the problem that the configuration for leading the electrodes, which are formed on the quartz crystal plate as the vibration plate, outside is complicated.